The invention relates to combustors, and more particularly to methods and apparatus for use with combustors employing premixed gas fuel/air mixtures.
Gas turbine engines are well known. A typical gas turbine engine has an annular, axially-extending flow path for conducting working fluid sequentially through a compressor section, a combustion section, and a turbine section. The combustion section mixes the working fluid, typically air, with fuel and ignites the fuel/air mixture.
Gas turbine engines typically burn fossil fuels. An undesired result of this combustion is the formation of nitrogen oxides, frequently referred to as NOx. Nitrogen oxides are pollutants capable of causing health and environmental problems. Government standards establish limits on the amounts of NOx that may be discharged into the air.
The rate of NOx production within the gas turbine engine directly depends on the temperature within the combustion chamber. In turn, the chamber temperature depends on the ratio of fuel to air in the mixture. The greatest chamber temperature and, thus, the greatest rate of NOx production result when the combustible fuel/air mixture has a stoichiometric fuel to air ratio, commonly referred to as xe2x80x9cstoichiometricxe2x80x9d mixture. A mixture having a fuel to air ratio that is less than stoichiometric, commonly referred to as a xe2x80x9cleanxe2x80x9d mixture, results in a lower temperature and a lower rate than that of a stoichiometric mixture. Increasing the leanness of a lean mixture results in an even lower temperature and an even lower rate of NOx production.
When using a lean mixture to attain a low rate of NOx production, it is desirable to mix the gaseous fuel and a large fraction of the air before they reach the combustor. This approach, commonly referred to as xe2x80x9cpremixingxe2x80x9d, uses a premixer to increase the uniformity of the mixture provided to the combustor. The fuel spends enough time within the premixer to ensure that it adequately mixes with the air. This time, typically referred to as residence time, might be about 1 or 2 milliseconds (xe2x80x9cmsecxe2x80x9d) but is more commonly about 4 msec. Without premixing, some regions within the combustor end up with an extremely lean mixture while others end up with a less lean, i.e., richer, closer to stoichiometric, mixture. Greater uniformity in the fuel to air ratio of the mixture results in a lower peak temperature within the combustor and hence less NOx.
However, even when premixing is employed, other considerations can effectively limit the leanness of the mixture. Mixtures that are too lean do not permit sustained combustion and ultimately result in a xe2x80x9cflame-outxe2x80x9d condition commonly referred to as xe2x80x9clean blowoutxe2x80x9d. Furthermore, lean mixtures having a slightly higher fuel to air ratio enable sustained combustion, but can result in oscillations in both the magnitude of the pressure and the heat release rate within the combustor. In some situations, the time relationship between these two oscillations is such that the oscillations in the magnitude of the combustor pressure cause an increase in the amplitude of oscillation in the heat release rate, and vice versa. This condition, commonly referred to as combustion instability, causes large oscillations in the magnitude of the pressure within the combustor. The repetition rate or frequency of these oscillations depends on the application. For industrial gas turbine engines, the frequency is typically within a range of about 100 Hertz (xe2x80x9cHzxe2x80x9d) to about 700 Hz, most often around 200 Hz. Thus, the period or duration of an oscillation is most often about 5 msec. The presence of combustion instability can lead to problems including engine damage. The possibility that combustion instability will occur could preclude the use of a fuel to air ratio that is only slightly above the lean blowout limit.
One technique for passive control of combustion instability involves injection of a secondary, or pilot, fuel mixture into a side wall of the combustor. U.S. Pat. No. 5,263,325 to McVey et al. discloses an example of this technique. However, the use of pilot fuel injection cannot sufficiently reduce combustion instability without also causing a significant increase in the rate of NOx production.
Several references disclose active control of combustion instability. For example, U.K. Patent Application GB 2239691A discloses an active control which uses a pressure transducer to measure pressure fluctuations in the combustion chamber and a servovalve to modulate the amount of fuel supplied for combustion U.S. Pat. No. 5,145,355 to Poinsot et al. discloses an apparatus that detects combustion instability and modulates the flow of fuel injected into the chamber as a function of the instability. U.S. Pat. No. 5,575,144 to Brough discloses a system that senses pressure pulses in the combustor, calculates a cancellation pulse to offset a predominate pressure pulse, and periodically extracts metered volumes of air from the combustor to produce a cancellation pulse. However, none of these references disclose a system for use with a combustor that burns a premixed gaseous fuel/air mixture.
U.S. Pat. No. 5,445,517 to Kondou et al. discloses an adaptive noise silencing system for a combustor. The system computes an anti-phase signal of a combustion noise and inputs the signal to a gas flow control valve, thereby producing a pressure variation in the gas fuel and realizing a pressure variation in the combustion chamber to suppress, by phase interference, the combustion noise. This system employs a mixing chamber between the gas flow control valve and the combustor chamber. However, all of the fuel flows through the gas flow control valve.
U.S. Pat. No. 5,349,811 to Stickler et al. discloses a system for reducing the formation of NOx pollutants. The system xe2x80x9cmodulate[s]xe2x80x9d the fuel delivery rate to the combustor to produce combustor air input flow oscillation and bulk flow oscillation within the combustor which enhances fuel/air homogeneity throughout the combustion chamber and reduces conditions favorable to the formation of NOx. However, Stickler does not disclose the use of such system with a combustor that burns a premixed gas fuel/air mixture, but rather as an alternative to premixing.
An object of the present invention is to provide an apparatus that provides a premixed gas fuel/air mixture having a temporally modulated stoichiometry, i.e., fuel to air ratio.
Another object of the present invention is to provide an apparatus for controlling the magnitude of combustion instability in a combustor that burns premixed gas fuel/air mixtures.
Another object of the present invention is to provide an apparatus for controlling the magnitude of oscillations in the pressure within the combustor in a gas turbine engine that burns premixed gas fuel/air mixtures.
Another object of the present invention is to provide an apparatus that can control the magnitude of combustion instability without significantly increasing the rate of NOx production within the combustor.
According to a first aspect of the present invention, an apparatus for use in a system having a gas fuel supply and a combustor includes a premixer in flow communication with the combustor, a first fuel line in substantially non-modulated fuel flow rate communication between the fuel supply and the premixer, and a second fuel line in modulated fuel flow rate communication between the fuel supply and said premixer. The modulated fuel flow rate communication is adapted to substantially reduce the magnitude of fluctuations in the magnitude of the pressure in the combustor.
According a second aspect of the present invention, a fuel system for use in a system having a gas fuel supply and a combustor includes a fuel actuator that receives gas fuel and provides, in response to a command signal from a controller, gas fuel at a modulated rate of flow, and further includes a premixer that receives gas fuel from the actuator and another source, such that the gas fuel received from the actuator at the modulated rate of flow represents only a portion of the fuel flow to the premixer. The premixer mixes the fuel with air, and provides the mixture to the combustor.
According to a third aspect of the present invention, a fuel system for use in a system having a gas fuel supply and a combustor includes a fuel actuator that receives gas fuel and provides, in response to a command signal from a controller, gas fuel at a modulated rate of flow, and further includes a pressure actuated fuel valve that receives gas fuel from the actuator at a first modulated rate of flow and provides, in response, gas fuel at a second modulated rate of flow, and further includes a premixer that receives gas fuel from the pressure actuated fuel valve, mixes the fuel with air, and provides the mixture to the combustor.
According to a fourth aspect of the present invention, a controller for use in a system having a fuel actuator that receives gaseous fuel and a command signal and provides, in response to the command signal, gaseous fuel at a modulated flow rate to a premixer that receives the modulated fuel flow from the fuel actuator, mixes the fuel with air, and provides the gaseous fuel and air mixture to a combustor, includes means for determining the actuation sought from the fuel actuator to cause the actuator to provide gaseous fuel at the modulated flow rate, and means for generating a command signal indicative of said actuation sought from the fuel actuator.
As used herein, the term modulation does not include quasi steady operation such as that universally employed within gas turbine engines to change the rate of fuel flow to the burner in response to a change in an engine operating condition. Those fuel rate changes typically occur very slowly, often over a duration of many seconds or even minutes. Contrast the modulated fuel rate of the present invention which changes at a faster periodic rate, typically but not limited to hundreds of Hertz (xe2x80x9cHzxe2x80x9d).
While systems employing modulation of the fuel flow rate to a combustor are known, until now, in regard to mixing prior to combustion, such systems fed all of the fuel to a mixing chamber through a flow control valve. While this may help ensure sufficient control authority and spatial uniformity of flow rate into the mixing chamber, it has the disadvantage that an excessively large valve may be required. Such a valve may not be capable of providing the desired modulation magnitude and frequency. However, it has been determined that all of the fuel flow to a premixer need not be modulated, i.e. only a portion of the fuel flow need have a modulated flow rate. In one embodiment, for example, the modulated fuel flow rate comprises as little as a minority of the total fuel flow to the premixer. Other other embodiments progressively smaller fractions of the total fuel flow may be employed, e.g., one third, one fifth, one tenth, one twentieth, on fiftieth. Progressively smaller fractions may for example offer the opportunity to use smaller, faster responding, lighter weight, or lower power comsumption actuator valves.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings.